Hertzsprung-Russell Diagram – Learn

Observing Stellar Spectra

As physicists and astronomers began to understand the link between a stars spectra and its features, they began to classify stars based on similar features and characteristics. The main features of spectra that are studied are the peak intensities of a blackbody curve and the absorption spectral lines. This information can be compared to known spectra on Earth to understand the properties and even the life cycle of a star.

  • The surface temperature of a star is determined by analysing the blackbody spectrum of a star. The blackbody curve will have a peak intensity that corresponds with a certain wavelength. The surface temperature of the star is inversely proportional to this wavelength according to Wien’s Law.
  • Comparing absorption spectra with known elements on Earth helps to determine the elements in the atmosphere of the star.
  • The thickness of the absorption lines is used to approximate the density of the gases in the stars atmosphere and its luminosity.

Spectral Classes

The spectral classes for stars are OBAFGKM, where O is the hottest and M is the coolest. Stars are classified based on the presence or absence of certain spectral lines. The changes between spectral classes is associated with the fact that different atoms are ionised at different temperatures. This will result in a certain range of absorption lines which may occur. Temperatures higher or lower than this will result in other transitions occurring  and being observed.

  • Higher temperatures can result in more transitions being observed as a result of extra electrons absorbing larger amounts of energy that was not previously available.
  • Lower temperatures can result in the disappearance of spectral lines as those electrons no longer get the energy required to make those transitions.

HR Diagram

Astronomers started graphing and plotting data that had been collected about stars. Two astronomers, Ejnar Hertzsprung and Henry Russell independently discovered a pattern when they plotted the luminosity of a star against its temperature or spectral class. (The luminosity of a star is a measure of the total amount of energy radiated by a star per second).They found distinct patterns and groups started to form. The graph they produced is known as the Hertzsprung-Russell diagram.

Star Groups in the HR Diagram

Stars on the HR diagram are not randomly distributed, they appear to be in several groups. The several groups are often referred to as:

  • Main sequence stars 
  • Red giants
  • White dwarfs
  • Blue giants

Most of the stars on the HR diagram form a band from the bottom right corner to top left corner. These stars are known as the main sequence stars. Main-sequence stars go from dull, red, cool stars at the bottom right to bright, blue, hot stars at the upper left of the diagram. There is a clear relationship between the brightness (luminosity) and temperature of a main sequence star. 

The red giants are relatively cool and very luminous. This means that they must be very large. Red super giants are even less common and are more luminous than red giant stars. Red giants are over 1000 times larger than our Sun.

White dwarf stars have low luminosity and relatively high surface temperatures and appear white. These stars are very small – about 1/1000 the size of our Sun. They are believed to be the collapsed remnants of stars like our Sun that have stopped nuclear fusion reactions.

Blue giants are very hot, luminous and have relatively short lives. These stars are different to main sequence stars in that nuclear fusion reactions in their core produce heavier elements.

A Stars Life Cycle

Stars go through an evolutionary life cycle. Clouds of gas and dust can form protostars before forming a main sequence star. The path that the star takes from this point depends on its mass. The diagram below illustrates the life cycle of various star types.

During the protostar stage gravity is pulling in more material from the surrounding gas and dust cloud and nuclear reactions have not begun. As the cloud of gas and dust collapses in on itself, pressure increases and the nuclear fusion of hydrogen into helium will begin in the core of the protostar – this is the ‘birth’ of the star.

The outward force caused by the pressure of gas in the star and the inward force of gravity must remain balanced for the star to remain stable as a main sequence star. Massive stars with a greater mass will have larger gravitational fields around them. This allows larger stars to have dense cores with more nuclear fusion reactions taking place. This results in the higher temperatures observed in larger main sequence stars.

Main sequence stars that are larger than our Sun burn quicker and have shorter lives than our sun – in the order of tens of millions of years. Small red stars with a mass of 0.1 solar masses burn cool and slow. They have much longer lives – billions of years. 

When the hydrogen fuel has been depleted the core of the star collapses. The excess gravity of the star elevates the core’s density and temperature. The layer of helium that has formed around the core is compressed to such an extent that the fusion of helium into carbon begins. The life of star from this point is dependent on its mass. 

In a star with sufficient mass, the star’s gravitational force will provide the density and pressure required for helium to fuse into heavier elements such as carbon and oxygen. This process results in the star becoming a red giant. Very massive stars are able to continue fusing elements all the way to the formation of iron. These fusion reactions provide the outward forces which prevent the star from collapsing under its own gravity. The formation of elements heavier than iron does not occur in stars and the existence of these heavier elements on Earth is due to supernovas or neutron star collisions that must have occurred before the formation of our solar system. 

Once these massive stars have used up all their nuclear fuel the core begins to collapse. This causes an increase in temperature and pressure, resulting in a supernova. Existing elements surrounding the core fuse into heavier elements, blowing off a large proportion of the outer layers of the star into space at great speed. An equal and opposite force compresses the core to such an extent that elements heavier than iron can be created. The remaining core of the star may collapse and electrons and protons will become neutrons.

The following diagram illustrates a range of pathways for different stars:

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